Conventionally, in order to enhance the durability of concrete products or iron and steel products, there have been employed coating agents containing a hydrolysis product of an alkoxide of a metal (e.g., silicon, zirconium, or titanium), the agents being capable of forming a highly durable film on the surfaces of the products (e.g., Japanese Patent Application Laid-Open (kokai) Nos. 2-69375 and 62-36045). However, the above coating agents involve problems in that film formation requires heating and the thus-formed film is not transparent.
Meanwhile, there have been proposed photocatalytic semiconductors which generate, through photoexcitation, active oxygen species exhibiting a strong oxidation power. As most organic substances are decomposed by means of the oxidizing effect (i.e., photocatalytic effect) of the active oxygen species, such a photocatalytic semiconductor has been employed as a photocatalyst in various articles for decomposition of malodorous substances, prevention of contamination with organic substances, and antibacterial purposes. The photocatalytic semiconductor exhibits the photocatalytic effect at its surface, and therefore, the semiconductor must be provided on the surface of a member requiring photocatalytic activity. In general, the photocatalytic semiconductor is provided on the surface of such a member by means of a simple process; specifically, a process in which the semiconductor is mixed with a binder, and the resultant mixture is applied onto the member. When an organic polymer serving as a binder is employed in combination with the photocatalytic semiconductor, the binder is oxidized and decomposed because of the photocatalytic effect, raising the need for employment of a binder which is not easily decomposed (e.g., a fluorine resin binder or a silicone resin binder) (Japanese Patent Nos. 2,756,474 and 3,027,739).
However, when photocatalytic semiconductor particles are incorporated into such a resin binder, the surfaces of the particles are coated with the binder, leading to problems that the binder prevents light or substances to be decomposed from reaching the particles, and lowers the photocatalytic effect of the particles. Also, employment of such a resin binder involves a problem that the resin must be cured through heating.
Japanese Patent Application Laid-Open (kokai) No. 9-40872 discloses a process employing, as a binder, an alkyl silicate condensation product of a relatively low molecular weight obtained through hydrolysis of a hydrolytic silicon compound. In this process, the binder can be cured at a low temperature, and lowering of the photocatalytic effect is suppressed.
However, in this process, hydrolysis must be performed at a pH of 4 or less or under alkaline conditions, and thus a solution containing the binder and photocatalytic particles has acidity or alkalinity. When the solution is applied to a corrosion-prone substrate such as iron or aluminum, the application method must be modified. In addition, the aforementioned condensation product cannot be employed as a binder in a neutral titanium oxide sol.
Japanese Patent Application Laid-Open (kokai) No. 9-40872 discloses a process employing a hydrolytic organometallic compound as a binder. However, this process involves problems in that hydrolysis of the compound is insufficiently regulated, and a titanium oxide sol aggregates and exhibits low storage stability.
Since aluminum alkoxide is very readily hydrolyzed by water in the presence of a small amount of a catalyst, a neutral sol can be formed from aluminum alkoxide. However, difficulty is encountered in forming a transparent and highly durable film from the sol, and the sol has a very short pot life. Meanwhile, Japanese Patent Application Laid-Open (kokai) No. 7-216552 discloses a technique for forming a highly durable film, but this technique involves problems in that a coupling agent which causes impairment of the film must be employed, and curing of the film requires heating.
A photocatalyst requires a binder which has a long pot life, which is not impaired by the photocatalyst, and which enables formation of a transparent and highly stable film.
WO 99/28393 discloses a technique employing a zirconium compound as a binder. Through this technique, a transparent and highly durable film having a skeleton of a zirconium inorganic compound can be formed. However, a heating step is required to cure the formed film.
An object of the present invention is to provide a transparent-film-forming composition which enables, through drying (without heating), formation of a transparent and highly durable film on the surface of a substrate. Another object of the invention is to provide a composite member which decomposes harmful organic substances through exposure to light; i.e., exhibits useful properties, including deodorization, antifouling, hydrophilic, and antibacterial effects.